remove/cli: drop unused validate commands - #1310

## Why

The docs sync from `sourcegraph/sourcegraph`
([`doc/_generated.push.sh`](https://github.com/sourcegraph/sourcegraph/blob/main/doc/_generated.push.sh))
does `cp -R -L doc/cli/** _clone/docs/cli/` — it only adds and updates
pages, never deletes. So when a command is removed from src-cli, its
reference page lingers here forever. Replaying the sync against `main`
shows zero drift in the 62 generated pages, but 36 files under
`docs/cli/references/` are never written by it.

This PR deletes the 28 that are stale. No redirects are added; none of
these pages are linked from anywhere in the site.

## Removed (28 files)

**Command removed from src-cli**

| Pages | Removal |
|---|---|
| `extensions/{index,copy,delete,get,list,publish}` |
sourcegraph/src-cli#1173 (extensions platform sunset) |
| `lsif` | sourcegraph/src-cli#1147 (→ `src code-intel upload`) |
| `scout` | sourcegraph/src-cli#1196 |
| `sbom`, `signature` | sourcegraph/src-cli#1273 (SBOMs/signatures no
longer published as of 7.0.2852; the how-tos already carry deprecation
notices) |
| `admin` | sourcegraph/src-cli#1309 (built for the appliance, which
never launched) |
| `validate` | sourcegraph/src-cli#1310 |

**Command never shipped**

`prompts` + 13 subcommand pages (#1155) documented
sourcegraph/src-cli#1168, which was closed unmerged.

**Stale duplicates shadowing generated pages**

`auth.mdx` and `codeowners.mdx` collide with `auth/index.mdx` /
`codeowners/index.mdx`: contentlayer flattens both to
`cli/references/auth`, and `allPosts.find` returned the stale flat file.
After this change `/cli/references/auth` and
`/cli/references/codeowners` render the generated group pages (verified
with `contentlayer build`).

## Kept

`search-jobs/{cancel,create,delete,get,list,logs,restart,results}` — the
subcommands exist, but `src doc` only emits a single `search-jobs.md`
because `searchJobsCommands` isn't in the `commanders` map in
[`cmd/src/doc.go`](https://github.com/sourcegraph/src-cli/blob/main/cmd/src/doc.go#L59-L66).
Fix belongs upstream.

## Verification

- `npx tsc --noEmit` passes
- `npx contentlayer build` generates 493 docs; `cli/references/auth` and
`cli/references/codeowners` now resolve to the `index.mdx` files
- No remaining links to the deleted slugs anywhere in `docs/` or `src/`

## Follow-up (upstream)

To stop this recurring, `_generated.push.sh` should mirror
`docs/cli/references/` (delete-then-copy) instead of copying over it.
Done in sourcegraph/sourcegraph#15529 (merge after
sourcegraph/sourcegraph#15528 and #1889).

## Amp threads

- [Stale command
docs](https://ampcode.com/threads/T-01a08410-86ca-72be-9928-2810e837fae1)

Co-authored-by: Amp <amp@ampcode.com>